External Insulation on New Build with Traditional Cavity Wall

CPS84

Hi All,

I have a query regarding EWI for a new build two storey detached house. I have acquired a house that was built to 1st fix stage in 2008 (i.e foundations, blockwork, roof, windows have all been completed). It has a traditional 300mm overall wide cavity wall construction made up of 100mm inner block, 60mm insulation, 40mm cavity and 100mm outer block. That is as far as it was built at the time and I’m now looking to finish it.

Because of some concerns about the wide op’s in the front of the house and the weight coming from the floors above etc, the house has a beam running along the front which turns down both gable ends to approx. half way. The beam is the full width of the 300mm wall construction with 270mm of concrete and 30mm insulation on the inner face. The beam is approx. 300mm high. The original design was for internal plaster boarded insulation(Kingspan K18 or similar) to be installed when completing the house however this was never finished.

Because of the thermal bridging issues that the beam will cause and the ingress of water that will occur (Water is already making its way in along the top of the beam), I am thinking about putting EWI around the house to achieve a good U-Value and protect against the ingress of water over through the top of the beam.

Is there any issues that may arise by putting EWI onto a traditional cavity wall like this for a new build? Will the EWI and render finish protect against the water penetrating through to the beam?

If the EWI is not feasible and internal insulation has to be done, is there any other ways of waterproofing the beam as simply plastering over will not do?

Regards

CPS

BryanF

from what you've stated above there is no issue with EWI assuming you pump the cavity, may is uggest you need to get details donw of all major junctions to make this investment worth while and to avoid further thermal bridges

CPS84

BryanF wrote: »

from what you've stated above there is no issue with EWI assuming you pump the cavity, may is uggest you need to get details donw of all major junctions to make this investment worth while and to avoid further thermal bridges

How important is it to pump the cavity? Is this to stop warm air looping in the cavity void?

BryanF

CPS84 wrote: »

How important is it to pump the cavity? Is this to stop warm air looping in the cavity void?

yes

Don't forget the details: window, eaves, rising walls, threshold, Penetrations, esb box, downpipes + how are you ventilating the house? Is air-tightness a factor your considering?

CPS84

BryanF wrote: »

yes

Don't forget the details: window, eaves, rising walls, threshold, Penetrations, esb box, downpipes + how are you ventilating the house? Is air-tightness a factor your considering?

So just to be 100% sure as to what would happen if the cavity void remains with EWI installed. Once heat penetrates through the 60mm insulation to the cavity, it will loop in the cavity void without the heat being slowly releasing back into the house when the heating is turned off, i.e no thermal storage. Could this also cause dampness in the cavity or any other issues?

But with the cavity filled, once heat penetrates through the 60mm insulation and filled cavity void towards the outer block leaf, with the EWI installed does this outer block leaf now work as a thermal store? The only question I have on this is once heat gets to the outer block leaf, does is essentially get trapped with insulation both sides of it, therefore not realising heat back into the house?

I’m just trying to fully understand the technical requirements of having to get the cavity pumped if EWI is installed and what issues it negates?

Also, I’m looking to install a MVHR system and will have air tightness taping etc throughout.

PROJECT K

Hi CPS84, pump the cavity - you really dont need air spaces between insulation layers if at all possible. While your end solution still wont be ideal as you will have the outer layer of blockwork separating the insulation layers, this blockwork will be maintained at a slightly elevated temperature after a few heating cycles so you arent losing significant heat into this thermal mass - thanks to the external insulation. I would advise you obtain a detailed condensation risk analysis from the EWI/insulation supplier just to make sure there is no risk of interstitial condensation. Also i would strongly advise you have detailed construction details drawn up for all the critical junctions particular to your house.

focuswrc

BryanF wrote: »

yes

Is air-tightness a factor your considering?

I'm in the middle of deciding whether to retrofit external insulation on a house with same construction as the OP.

BryanF, I read a post on another forum where a poster said he had been hung up on ensuring breathability to the extent of actually making a spiking tool for the blocks of EPS but over time and many installations realised that this didn't matter as long as the house had a really high level of air tightness. Does this make sense?? I don't fully understand why good airtightness negates the need for breathability. Also if you make a house really airtight is it a neccesity to install MVHR or similar? Hope this isn't too off topic, I thought it had relevance given the contruction method..

BryanF

Can we have some context please?

Insulation is important

As is not letting the heated air piss out through the fabric

focuswrc

BryanF wrote: »

Can we have some context please?

Insulation is important

As is not letting the heated air piss out through the fabric

Yea, I didn't mean in a general context. Of course air tightness is important but why does a relatively high level of air tightness negate the need to use breathable materials? For EWI I could use rockwool which is breathable or EPS which is not so breathable or does it matter? I'm wary that as it's a retrofit, if I don't get really high levels of airtightness just due to the way the house is built etc that if I don't use a breathable insulation, I'll have condensation / damp issues... Hope this makes more sense...

BryanF

What is the current wall build up & age of the house?

Breathability in building fabric has little to do with air-tightness.

focuswrc

Sorry Bryan, I've reread the post I was talking about again. All the poster was saying was that with regard to ewi, breathability was not that important i.e. in relative terms you can use whatever material you want EPS, rockwool, whatever! So just do the ewi with whatever insulation you want and instead spend time and effort getting a good level of airtightness. I'm a newbee to this so easily confused!

I was thinking of using the "Baumit EWI System open" for the retrofit of the house we're buying. House is standard 300mm cavity construction (100 block, 100 cavity, 100 block). Any thoughts?

BryanF

If you can pump a 100mm cavity, that's a good start, if your spending x on Ewi what have you allowed for windows and all the junctions: floor, wall, eaves chimney, etc. I'm not knocking Ewi but where there is a cavity to fill there might be other thermal priorities